I upgraded a soldier to level 28, then it was suggested that I raise his rank. What it is?

You can raise the rank of your troops after they reach level 28. If you raise the rank of your soldier, then he will get a new set of officers absolutely free, and the performance and skills of the soldier can be upgraded by up to 20 levels.Ranks are available for both regular and for elite troops. You are given the opportunity to raise the rank of elite troops only after you have acquired all possible upgrades for the soldier.